Many therapeutic agents for such diseases have undesirable side effects (e.g., chemotherapeutic agents) or, for reasons such as in vivo stability, transport, or other pharmacokinetic properties, are difficult to provide at a sufficiently high concentration in the target tissue or for a sufficiently long duration to allow maximal therapeutic effect in the target tissue. Accordingly, there is a need for methods and compositions that increase concentrations of therapeutic and diagnostic agents in target organs or tissues such as the brain, ovary, liver, or lung.